Tony Vargas
Legend
Good idea. Or you could give attackers advantage until the rudely-awoken character makes a CON save (repeated at the start of each of his turns), the save could be harder if the character was exhausted or badly wounded or tapped out of spell slots before he went to sleep...If you really want to make resting time more dangerous (as I assume you do by asking about a rule to encourage people not to rest in armor) without being unduly punitive to specific classes, you could implement a wake-up penalty where each character is considered to be exhausted for the first 1d4 minutes after waking up (note, the character would not actually accrue any levels of exhaustion; they would simply be treated as having one level of exhaustion).